Disposal of waste plastic and recovery of valuable products therefrom

ABSTRACT

A method of disposing of waste plastic without polluting the environment by passing the plastic to a reactor, heating the plastic in the presence of a gas to at least the decomposition temperature of the plastic, and recovering decomposition products therefrom. The preferred embodiment utilizes a heated inert carrier gas as the source of heat.

ilnited States Patent 1191 Banks et a1. a

1 DISPOSAL OF WASTE PLASTIC AND RECOVERY OF VALUABLE PRODUCTS THEREFROM[75] Inventors: Michael E. Banks, Torrance; Walter D. Lusk; Robert S.Ottinger, both of Hawthorne, all of Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Department of Health, Education and Welfare,Washington, DC.

[22] Filed: June 21, 1971 [21] Appl. No.: 154,861

[56] References Cited UNITED STATES PATENTS 2,577,632 12/1951 Roctheli..20l/36X 1451 Aug. 13, 1974 2,705,697 4/1955 Royster 201/36 X 3,220,79811/1965 Cull et a1. 23/155 3,305,309 2/1967 Woodland et a1. 23/1553,582,279 6/1971 Beckman et a1... 201/25 X 3,589,864 6/1971 Ezaki 23/1543,650,830 3/1972 Mathis 201/25 3,668,077 6/1972 Ban 201/32 3,716,3392/1973 Shigaki ct al. 423/488 X Primary ExaminerEdward Stern Attorney,Agent, or Firm-Holman and Stern 5 7] ABSTRACT A method of disposing ofwaste plastic without polluting the environment by passing the plasticto a reactor, heating the plastic in the presence of a gas to at leastthe decomposition temperature of the plastic, and recoveringdecomposition products therefrom. The preferred embodiment utilizes aheated inert carrier gas as the source of heat.

5 Claims, 5 Drawing Figures will . fiwzoxw $250 29296 mvsw'rons MICHAEL5.. BANKS WALTER D. LUSK ROBQRT' S. OTTINGER DISPOSAL OF WASTE PLASTICAND RECOVERY OF VALUABLE PRODUCTS THEREFROM BACKGROUND OF THE INVENTIONThis invention relates to the disposal of waste plastic withoutpolluting the environment, and more particularly to a method ofthermally decomposing waste plastic and recovering the decompositionproducts thereof.

Various methods have traditionally been used for the disposal of solidwaste material over the years. These methods can be broadly classifiedin two categories, namely destruction by burning and disposal into theenvironment by either dumping into the sea or burying on land. Each ofthese methods in general contributes to the pollution of the environmentand is, therefore, undesirable. An increasingly large precentage ofsolid wastes is occupied by various plastics which have rapidlydisplaced other materials in a modern technological society. Thedisadvantages associated with disposal of plastics in the traditionalmanner are manifest. If plastics are merely dumped into the ocean orother waterways, or are buried or used for so-called sanitary land fill,they represent pollution in a very basic sense. These materials are notbio-degradable and, therefore, remain in an unchanged state. On theother hand, if plastics are incinerated they form products which, whenreleased to the atmosphere, represent'a grave danger to the community.These combustion products, depending on the type of plastic involved,include hydrogen cyanide, hydrogen chloride gas, various nitrogenoxides, and the like. Thus, it is abundantly clear that methods andmeans must be developed for the disposal of plastic waste withoutpolluting the environment.

Accordingly, it is a primary object of the present invention to providea method of disposing of waste plastic without polluting the environmentwhich is free of the aforementioned and other such disadvantages.

It is another primary object of the present invention to provide amethod of disposing of waste plastic without polluting the environmentand recovering certain byproducts of the disposal.

It is a further object of the present invention to provide a method ofdisposing of waste plastic by thermal degradation without polluting theenvironment and recovering the degradation products thereof.

It is still another object of the present invention to provide a methodof disposing of waste plastics either of a single type or a mixture ofplastics, without polluting the environment by thermal degradationthereof and recovery of degradation products.

It is yet another object of the present invention to provide a method ofdisposing of waste plastics without polluting the environment by heatingthe same in the presence of an inert carrier gas to at least thedecomposition temperature of the plastic and recovering thedecomposition products thereof.

DETAILED DESCRIPTION OF THE INVENTION This invention will be betterunderstood, and objects other than those set forth will become apparent,after reading the following detailed description thereof. Suchdescription refers to the annexed drawings presenting preferred andillustrative embodiments of the invention.

In the drawings:

FIG. 1 is a schematic elevational view of a semicontinuous reactorsuitable for use in one embodiment of the present invention;

FIG. 2 is a schematic elevational view of a combustion reactor suitablefor use in another embodiment of the present invention;

FIG. 3 is a schematic flow diagram of the various steps according to apreferred embodiment of the present invention;

FIG. 4 is a flow diagram of the steps according to another embodiment ofthe present invention; and

FIG. 5 is a flow diagram of the steps according to another embodiment ofthe present invention.

In its most basic aspect, the present invention involves a method ofdisposing of waste plastic without polluting the environment bythermally degrading the same in a reactor and recovering certaindecomposition products thereof which have economic value whilecombusting other decomposition products for the recovery of heat value.The present invention involves the disposal of waste plastics which fallinto various categories. Basically, the importance of the presentinventive method in relation to the various categories of plastic isdictated by the commercial importance of the types of plasticthemselves. For instance, it has been found that the typical plasticswaste for disposal comprises, on the average, approximately 50 percentpolyvinyl chloride, approximately 30 percent polystyrene, and theremainder, or approximately 20 percent, various other plastics such aspolyethylene, polypropylene, polyesters, polyacrylics, and the like.Thus, for the most part, the waste plastics fall into three importantcategories, namely, poly(halogenated hydrocarbons), poly(straight chainolefins), and poly(vinyl aromatics). The representative, and mostcommercially important members of these three categories are polyvinylchloride, polyethylene, and polystyrene, respectively. Accordingly,these three particular plastics will be discussed herein as exemplarymembers of the categories embraced by the instant invention and shouldnot be considered limiting thereof.

It has been found that when polyvinyl chloride is heated to at least thedecomposition temperature thereof, hydrogen chloride and a hydrocarbonmixture are obtained as the decomposition product. When polystyrene isheated to at least the decomposition temperature thereof, the onlydecomposition product is monomeric styrene. If a mixture of plastics,such as that typically found in waste plastics, is heated to at leastthe decomposition temperature of all the constituents thereof, thedecomposition products will comprise hydrogen chloride, monomericstyrene, and a mixture of hydrocarbons. These relationships, of course,are based on the thermal decomposition of these materials in anon-oxidizing atmosphere. If mixed plastics are heated to at least thedecomposition temperature thereof in the presence of air, thedecomposition products will be hydrogen chloride and heat.

ln the thermal decomposition of these plastics in a non-oxidizing, orinert, atmosphere, a particular predominant distribution ofdecomposition products was found. The primary products produced atequilibrium for the three plastic types vary with carbon-hydrogen ratioin the original plastic material as can be seen in Table I.

TABLE I zene and hydrogen chloride. This is similar to the polystyreneand polyethylene decomposition, except that polyclhylcnc polysyrencPolyvinyl Chloride no hydrogen chlor de can be formed in the first twosystems. A decomposition path analysis was run on polyvi- H: Q B 5 nylchloride excluding benzene from forming. This run i1 z u r. LH" d h. b dh I s d C2HI CH4 CHH" in tea e a c orme su stttute et y enes an to uh".1 ene are formed. Another run deleting the above as pos- Primary thermaldecomposition products at equilibrium for the three slble pg'oductsyielded ethyl benzene as the next level plus'lic types. of feasibleproducts.

TABLE II *POLYSTYRENE POLYETHYLENE POLYVINYL CHLORIDE a a a s CH3CH3 {lg trans CH3CHCHC6H, CH CH CH canc u, a cis Cl-ncHCt-icn-i, CH:,CBH,, mpgortho CH C H CHCH c ii c ii, HCl 5 meta CH,C,,H,CHCH ortho Cli C Hfll-lcrt c n age para CH3CBH,CHCH meta CH,C,H,CH; CHClCCl para cH,c,i-l,cH Hcc|,cci ra cata 4 am P4 CH CHC H C H, l-lCl at 800C. in place of 500C.

The relative amounts of the various products naturally vary as afunction of temperature and pressure; the amounts of the unsaturatedaliphatics increasing as temperature increases and decreasing withincreasing pressure. Benzene appears to be particularly favored in thepolystyrene and polyvinyl chloride systems. The polystyrene isstoichiometric to benzene, andthe polyvinyl chloride is alsostoichiometric to benzene when hydrogen chloride is formed.

The non-oxidizing atmosphere is provided by any inert gas which alsoserves as a carrier gas. While the present invention is not limitedthereto, it has been found that nitrogen and steam are the two preferredinert carrier gases for use in this aspect of the present invention.

The polystyrene thermal decomposition equilibrium results indicate thatbenzene is in equilibrium with styrene at 800C. and 1 atmospherepressure. This equilibrium mixture represents a minimum of the relativesystem free energy, or in other words, the most probable distribution ofspecies at equilibrium. When benzene is not allowed to form, thereaction yields a new product distribution which can be seen in TableII. The new product distribution represents a higher system free energy,which may be interpreted to mean that these products are similar to theactual intermediates in the benzene from polystyrene reaction. Thepossible intermediates include toluene, the cis-transl-phenyl-lpropylene and the ortho-meta-para methyl-styrene.

Polyethylene equilibrium results, like polystyrene results, showconversion of the given plastic to benzene at 500C. and 1 atmosphere.However, the feasible reaction intermediates are for the most partdifferent as shown in Table ll. Toluene is the only species that appearsin both cases. In addition to toluene. the feasible intermediatesinclude ethane, propane, ethyl benzene, and ortho-meta-para dimcthylbenzene. The major coproduets of the thermal decomposition of polyvinylchloride at 500C. and 1 atmosphere pressure are ben- Feasible ReactionPath Species Thermal Decomposition 500C, l ATM A kinetic analysis ofthermally decomposing polystyrene, polyethylene, and polyvinyl chloridewaste plastics was made by determining the physical and chemicalproperties of the reactants and the decomposition mechanism. The resultsare presented herein as concentrations of reaction products and nitrogengas, which was the heat source, as a function of time. From these data,the reactor geometry and heat requirements were defined.

Since the kinetic behavior of a chemical system is strongly dependentupon the physical and'ehemieal properties of the reactants, in order torealistically treat these time dependent characteristics of thepolystyrene decomposition reaction, certain physical and chemicalproperties of waste polystyrene were determined. Sytrene may bepolymerized by free radical initiators in several ways. Generally, thereaction is CH=CH2 CH CH The polymerization may be in solution or in anemulsion system which latter term as used herein includes .producedpolystyrene. Styrene polymerizes to give a head to tail sequence in thepolymer chain. Of course, the reaction conditions and choice of catalystcan be controlled to produce a polymer having the desiredcharacteristics. In determining the kinetic behavior of the system, thefollowing properties of polystyrene were determined:

1. Polystyrene waste plastic contains thermally active cites (weaklinks), which are randomly distributed within each molecule;

2. Waste polystyrene is heterogeneous with respect to initial chainsize;

3. The reactant plastic can be characterized by an initial most probablemolecular weight distribution;

4. Plastic fed to the reactor is selected at random, that is, nodistinction is made on the basis of degree of polymerization, molecularweight, and the like;

5. It is homogeneous with respect to monomer type, i.e., polystyreneonly.

Initially, the decomposition of commerical polystyrene occurs by arandom mechanism due to weak links formed in the polymerizationreaction. When a weak link is broken, a number of monomers are splitoff. This phase continues until the weak links are exhausted. Duringthis rapid initial period, inhibitors are produced to give rise to aninduction period which is very pronounced at 352C. and below. Above402C. the induction period is not experimentally evidenced for eitherunfractionated or fractionated samples.

A depolymerization reaction occurs at the end of the induction period bythe following mechanism:

chain end initiation reaction, transfer reactions, propagation reaction,and termination reaction. Transfer reactions are at a trace level fortemperatures between 427C. and 727C. which means high styrene monomeryield with 100 percent selectivity.

The grams of styrene produced per gram of polystyrene as a function ofthe solid residence time (that is, the residence time of the solid wasteplastic material), were determined for a broad range of temperatures. Asexpected, the higher the temperature, the faster the rate. The increasein rate, however, must be balanced against heat requirements as apractical matter. The heat necessary to maintain an isothermal reactoris supplied by hot nitrogen gas which is preheated to 1,500K., or1,227C. At 92 percent conversion, the reactor requires a total of 15.5grams of nitrogen per gram of styrene when the initial nitrogen is at1,500K.

This may be compared with 82.0 grams of nitrogen per gram of styrene at1,000K., or 727C., initial nitrogen temperature. The quantity ofnitrogen at 1,500K. required per gram of styrene produced increases withreactor temperature at fixed conversion. The economies in nitrogen gasat the higher conversion is due to the fixed amount of nitrogen requiredto bring the waste plastic from ambient temperature to reactortemperature. I

The reactor residence time-heat requirement tradeofi, the requiredconversion, and the physical properties of unreacted waste plasticdictate the necessary solid residence time and as such, the totalvolume. From this, an economical reactor system and associated processwere designed and used to estimate the costs for processing each poundof waste polystyrene.

As expected in a kinetic study, the physical/chemical properties ofwaste polyethylene are important in characterizing its time dependentnature. Although a simple linear structure is present almost exclusivelyin low pressure polyethylene, it has been found that high pressurepolyethylene has its linear structure interrupted by -and a small amountof unsaturation. The molecular weight distribution of high pressurepolyethylene generally ranges between 22,000 and 25,000, while lowpressure polyethylene has a molecular weight distribution of from as lowas 20,000 to as high as 3 million. Additional physical properties ofwaste polyethylene are equivalent to the ones listed for polystyrene.The decomposition reaction mechanism for polyethylene, however, proceedsin a different fashion from that of polystyrene. Polystyrene degradesinto monomeric styrene between about 427C. and about 727C. Reactionproducts resulting from polyethylene contain little monomer butprimarily paraffms with up to 50 carbon atoms depending on thetemperature. The degradation reaction for temperatures between 387C. and437C. appear to be of zero order over a large range of percent weightloss. The mechanism probably consists of splitting off large molecularfragments in rapid succession once the chain is initiated.

An analysis was made to characterize the polyethylene system. Thequantity of product produced versus residence time with a polymer of 822average degrees of polymerization varies strongly with temperature.Temperatures between 412C. and 437C. were found to be practical reactortemperatures, considering heat input requirements and residence times.The rate was found to be affected by the average degree ofpolymerization, but inasmuch as it is impossible to determine wastepolyethylene average degree of polymerization a priori, its effect mustbe considered parametrically in designing the reactor. The effect ofaverage degree of polymerization over a range of 571 to 3,000 units wasexamined and the heat versus rate or conversion was studied for thereactor design. The product distribution for a polyethylene thermaldegradation system was calculated at about 437C. and is set forth inTable III. The relative concentrations of species is approximatelyconstant at each conversion.

Polyethylene Thermal Decomposition 437C. reactor temperature 3000 secondresidence time 1.0 g/sec polyethylene feed Polyvinyl chloride isessentially a linear polymer of head to tail configuration. A smallamount of branching is present which is probably composed of carbon andchlorine atoms. The general properties of waste polyvinyl chloridematerial are the same as those discussed for polystyrene.

The thermal degradation of polyvinyl chloride is primarily adehydrochlorination reaction. After a short initial period, hydrogenchloride is evolved under amechanism of approximately three-halves orderin the fraction of undegraded units. Following hydrogen chlorideevolution, a secondary decomposition occurs which yields numerousorganic products as shown in Table IV.

Polyvinyl Chloride Thermal Decomposition 437C. reactor temperature 255seconds residence time 1 g/sec polyvinyl chloride tial 1.2 grams pergram of waste plastic polymer necessary to raise the temperature fromambient temperature to 450C.

Attention is now directed to FIG. 1 wherein there is shown asemi-continuous moving-bed reactor designed for use with the method ofthe present invention. The reactor, generally designated by the numeral10 is operated isothermally by supplying hot inert carrier gas such asnitrogen or steam at various points throughout the reactor. The gasenters inlet 12 and is conducted throughout the reactor by'conduit 14 tobe emitted at a plurality of locations by suitable injector means 16.Waste plastic is fed continuously from above at intake 18 and iscontacted by the hot gas throughout the reactor. Any unreacted solidmaterial is removed in a batch fashion at outlet 20. During removal ofunreacted material, auxiliary reactors are switched on-stream. Thedegradation products and the inert gas are drawn off at outlet 22 forfurther processing to be discussed in more detail hereinbelow. Thespecific dimensions of the reactor for each of the individualdecomposition systems is summarized in Table V.

TABLE V Reactor Geometry For a Waste Plastic/Nitrogen DecompositionReactor* Reactant Temperature Per Cent Radius Length Volume Converted(cm) (cm) (em Polyvinyl Chloride 723K I0 299x10 Polystyrene 873K 90% 303.25Xl0 Polyethylene ADP 822 710K 75% 30 5.37 l0-" Polyethylene 700K ADP571 75% 30 230 6.5l 10 ADP 900 75% 30 500 1.41Xl0 ADP 3000 50% 30 5651.60Xl0 The effect of temperature on the hydrogen chloride and on thehydrocarbon reaction were examined to determine feasible reactionconditions, the net result is that the reaction is commercially feasibleabove about 402C. Selecting 450C. as the nominal case, the hydrogenchloride and hydrocarbon product evolution was examined as a function ofsolid residence time. Here the hydrogen chloride is the predominantproduct at all residence times. The overall reaction is exothermic dueto the predominance of the exothermic hydrogen chloride stage over theendothermic organic evolution stage. As such, no nitrogen is requiredafter the ini- 0 All data is based on 1.25 million pounds of plasticannually 0 ADP" signifies average degree of polymerization The nitrogeninput was held at 1500K Kinetic analyses were made of the systems forthermally degrading waste plastics according to the present inventionusing both nitrogen and steam as the inert carrier gas. A determinationwas also made whether The waste plastic/steam systems without catalystsgave kinetic results which were similar to the waste plastics/- nitrogensystems in that the product distributions were essentially the same foreach system. It should be noted that steam has a higher heat capacitythan nitrogen, thereby reducing the carrier gas concentration necessary,and hence reactor gas volume, for the reaction. For example, thepolyvinyl chloride decomposition reactor operating isothermally at 723K.requires 0.9 mole at l,5001(. as compared to 0.7 mole of steam at1,500K. A nitrogen/steam comparison was calculated for a variety ofthermal decompositions systems and conditions, the results of thecomparison being set forth in Table VI.

TABLE VI Amount of Indicated Heat Source* Necessary for the SpecifiedConversion Nitrogen Steam Polystyrene, 823K Grams Grams Converted 55%9.35 4.47 50% 9.13 4.38 25% 7.96 3.82

Polyvinyl Chloride, 673K Grams Grams Converted 90% 1.14 .58 75% 1.04 .5550% 1.00 .53 Polyethylene, 685K ADP 822 Converted 75% 1.75 .86 50% 1.51.74

* O 1500K heat source temperature 1 gram plastic A fourth wasteplastic/steam system, having an equal weight mixture of polystyrene,polyethylene, and polyvinyl chloride, was run. The product distributionwas determined and is set forth in Table VII. The reactor design wasthat shown in FIG. 1, the geometries being set forth in Table VIII.

TABLE VII Mixture Reaction Products At 700K and 4400 Seconds SolidResidence Time TABLE vm Reactor Geometry for a Waste Plastic/H 0Decomposition or The steps in one embodiment of the method of theinstant invention can be easily followed by reference to FIG. 3. Thepolyvinyl chloride waste is fed into a semicontinuous stainless steelreactor 24 of a type shown in FIG. 1. Nitrogen, which is used as aninert heat carrier, is preheated to l,230C. in a direct fired heaterusing oil/gas combination burners. The hot nitrogen stream is fed to thereactor 24 where thermal degradation of the polyvinyl chloride isaccomplished at a constant temperature of 450C. The overall system isexothermic, with the heat carried by the nitrogen being used to elevatethe temperature of the polyvinyl chloride feed. After leaving thereactor, the gaseous reaction products are sent to the scrubbing systemwhich is capable of removing 99 percent of the hydrogen chloride fromthe reactor effluent. The scrubbing system includes graphite humidifyingtower 26 and primary falling film absorber 28 where the organic phase isremoved. From the primary falling film absorber 28, the effluent goes tosecondary falling film absorber 30 from which 18 to 20 Baume acid isremoved, with a portion of the product going to tertiary falling filmabsorber 32. The material from the tertiary falling film absorber 32 iscycled back to the secondary falling film absorber 30 to be withdrawn asthe product acid of 18 to 20 Baume. If desired, an additional strippersystem can be used to produce anhydrous hydrogen chloride. This systemwould include falling film stripper 34, water condenser 36, brinecondenser 38, and entrainment separator 40. Bottoms cooler acid can bedrawn from falling film stripper 34 at 42 and 21 percent stripper storedin suitable means such as a tank 44 which supplies make-up liquor totertiary falling film absorber 1n the method for thermal degradation ofscrap polystyrene, a series of steps as outlined in FIG. 4 is employed.The polystyrene is fed into stainless steel reactor 46 which is of thetype shown in FIG. 1. Nitrogen is used as the inert carrier gas and isfed into the reactor at about 1,230C. so as to provide a degradationtemperature of about 600C. The efiluent from reactor 46 passes to heatexchanger 48 where the gaseous reaction products are cooled to about250C. by heat exchange with the nitrogen recycle from the refrigerationstep. From the heat exchanger 48 the gaseous reaction products are sentto secondary heat exchanger 50 where they are further cooled with waterto about 50C. Refrigeration is used to cool the stream to about 20C. inrefrigerator 52, removing all but 0.6 percent of the styrene monomerfrom the gaseous nitrogen. The stream then goes to phase separator 54and an inhibitor is added to the liquid monomer before it is pumped tothe product storage tank 56. The nitrogen leaving the refrigerator 52via the phase separator 54 is recycled to the direct fired heater 58,cooling the effluent reactor gases on the way. In the direct firedheator step, makeup nitrogen is added to the recycled nitrogen stream,now at 460C., and heated up to 1,230C. using oil/gas combinationburners.

When degrading mixed waste plastics, according to the instant invention,the plastics are charged to a semicontinuous stainless steel reactor 60in FIG. which also is of the type shown in FIG. 1. Nitrogen, again usedas an inert heat carrier, is preheated to 1,230C. in a direct firedheator 62, using oil/gas combination burners. Degradation of the wasteplastics mixture is accomplished at a constant temperature in thereactor 60 of 600C.

After leaving the reactor, the gaseous decomposition products are cooledby heat exchange with incoming nitrogen being sent to the direct firedheator for preheating in heat exchanger 64. The stream temperature isthen lowered to 90C. with a water spray quench at 66 which removes thehydrogen chloride and water from the stream. The condensed hydrogenchloride/- water stream is sent through a phase separator 68 forrecovery of heavy hydrocarbons and then on to a hydrogen chloridedistillation column 70 through preheator 72. 1

The gaseous stream leaving the quench 66 is cooled to 50C. with a coldwater heat exchanger 74, sent to a phase separator 76 to remove organicsother than styrene, and then refrigerated to C. by refrigerator 78,which removes all but 0.6 percent of the styrene monomer from the gasstream. After passing through phase separator 80, the condensed styrenestream is further purified in a vacuum finishing column 82, treated withan inhibitor, and sent to storage.

When using steam instead of nitrogen as the inert carrier gas, lowpressure steam is sent through the direct fired heater and heated to1,230C. prior to being sent to the reactor. Basically, the same methodsteps are used for the recovery of the degradation products.

In another aspect of the present invention, the mixed plastic wastes arethermally degraded in the presence of an oxidizing atmosphere, thepreferred such atmosphere being air. In order to prevent the formationof hot spots in a combustion reactor, the design of the same was made asindicated in FIG. 2. The reactor, generally indicated by the numeral 84,has a waste plastics inlet 86 and an air inlet 88. The oxidationproducts are drawn off at 90. The reactor system is adiabatic with airentering at ambient temperature. The product distribution, reactor exittemperature, and air input was calculated for systems of polyvinylchloride, polyethylene, polystyrene, and a mixture of equal parts byweight of the three plastics. The results of the determination are shownin Table IX.

All data Based on 1 Gram of Plastic The result of the combustionaccording to this aspect of the present invention is to completelycombust the organic material with the subsequent harnessing of the heatenergy produced. The hydrogen chloride is recovered following the methodshown in FIG. 3, the only difference being that the reactor is of anadiabatic type shown in FIG. 2 rather than the semi-continuous reactorof FIG. 1. The need for a hot nitrogen or other inert carrier gas is, ofcourse, not necessary.

Having now described in illustrative and preferred embodiments of thepresent invention in sufficient detail to permit a completeunderstanding of the various aspects of the invention, it should beapparent to those reading this specification that the objects set forthat the outset hereof have been successfully achieved.

What is claimed is:

l. A method of disposing of waste plastic without polluting theenvironment, said waste plastic being a mixture of polyvinyl chloride,polystyrene and polyethylene, said method comprising passing saidplastic to a reactor, contacting said plastic in said reactor in acounter-current flow with an inert carrier gas, said gas entering saidreactor at a temperature sufficient to heat said plastic to at least thedecomposition temperature thereof, heating said plastic to a temperatureof about 600C. by said gas to thereby produce reaction productscomprising hydrogen chloride, monomeric styrene, and mixed hydrocarbons,and recovering said decomposition products.

2. A method as defined in claim 1, wherein said inert carrier gas enterssaid reactor at about 1,230C.

3. A method as defined in claim 1, wherein said gas is selected from thegroup consisting of nitrogen and steam.

5. A method as defined in claim 1, further comprising the steps ofcooling the reactor effluent to a temperature below the boiling point ofstyrene and separating the resulting liquid styrene from the inertcarrier gas.

2. A method as defined in claim 1, wherein said inert carrier gas enterssaid reactor at about 1,230* C.
 3. A method as defined in claim 1,wherein said gas is selected from the group consisting of nitrogen andsteam.
 4. A method as defined in claim 1, further comprising the stepsof recovering said hydrogen chloride acid by scrubbing the same from thereactor effluent and carrier gas, and burning said hydrocarbons for theutilization of the heat produced thereby.
 5. A method as defined inclaim 1, further comprising the steps of cooling the reactor effluent toa temperature below the boiling point of styrene and separating theresulting liquid styrene from the inert carrier gas.